IRON K-ALPHA LINES FROM X-RAY PHOTOIONIZED ACCRETION DISKS

The properties of the iron Kalpha line emitted by an accretion disc illuminated by an external X-ray source are calculated for different values of the disc accretion rate m, and for two different source geometries: a point source located on the disc axis and an extended source above the innermost part of the disc. We find that for large values of rh the matter can be significantly ionized, and the iron line equivalent width can reach values as high as 250 eV for the point source, and up to about 400 eV for the extended source; the line centroid energy, in the emitting rest frame, is significantly higher than 6.4 keV, the value for neutral iron. A further increase of m leads to a strong decrease of the line intensity, because the iron becomes fully stripped in the inner region of the disc. The line profiles in the Schwarzschild metric are also calculated, and for the point source they appear much more complex than those obtained assuming neutral matter.